1. Field of the Invention
This invention relates to a system for measuring lengths or angles comprising a scale member provided with absolutely coded scale elements and incremental scale elements, a scanning unit, which is movable in a measuring direction relative to the scale member and includes reading means for generating position-dependent signals in response to the reading of the scale elements, and an evaluating unit for determining the absolute position of the scanning unit relative to the scale member in response to the position-dependent signals generated by the reading means.
2. Description of the Prior Art
In known absolute measuring systems of that kind, code features arranged in numerous parallel tracks are employed for the absolute measurement and may be encoded, e.g., in accordance with the so-called gray code. Separate reading means are required for each track and the signals generated in response to the tracks read in parallel directly indicate the instantaneous position of the scanning unit relative to the scale member. If such absolute measuring systems are required to measure with an accuracy not higher than 0.01 mm over a length of only 500 mm, 16 tracks and associated scanning means will be required so that the expenditure for the scanning means and for the scale member, which has a large width, will be high and a canting of scanning means relative to the scale member may result in an inaccurate reading. It is also known to provide that track on the scale member which has the highest resolution with an incremental scale and to scan that scale by methods known for incremental measuring systems, e.g., in that said fine track is projected onto a spaced apart scanning unit so that a higher resolution is achieved, which is still much lower than in the known incremental measuring systems.
DE-C 26 19 494 discloses for an absolute measurement of lengths or angles a system in which the code words for indicating an instantaneous position are encoded by a serial rather than a parallel code and are consecutively arranged on the scale member. Each code word is provided with an initial mark, which is the same for all code words. Reading is effected by a multiline detector, which in each position relative to the scale can detect at least one code word so that a certain absolute measured value is defined by the currently detected code word and by the position of the initial mark relative to the detector. That measuring system has only theoretical utility because a satisfactory indication can be obtained substantially only when the scanning unit is at a standstill relative to the scale member. Besides, the resolution which can actually be achieved is much lower than in incremental measuring systems and the entire structure is extremely expensive because the projection of a segment of the scale member onto the scanning unit involves a magnification.
Another basic disadvantage of all known systems for absolute measurement resides in that they can be used only with specially adapted evaluating units and for this reason cannot be adapted to existing measuring and evaluating systems, such as control systems, which have inputs only for countable and reference signals generated by incremental measuring systems.
Incremental measuring systems distinguish by having scale members which can easily be made and have a narrow scale having a high resolution and by having relatively simple scanning units. Such incremental measuring systems can be used in known manner for absolute measurements if separate tracks contain reference marks in positions which are related to the incremental scale by an absolute value; said reference marks can be identified by various methods so that the absolute position of any point of the scale member can be determined by an incremental measurement of the distance from that point to the last identified reference mark. In separate tracks, code words may be provided, which are associated with and identify respective reference marks and said code words can be read by separate reading means although this will increase the overall expenditure. The code words can be read by the same scanning methods as the respective scale or by different scanning methods, e.g., like tape recordings. It is also known to provide so-called distance codes for the reference marks if each reference mark is spaced different distances from the two adjacent reference marks and can be identified by the measurement of said distances. An example of a distance code is disclosed in DE-C 24 16 212. The use of identifiable reference marks has the disadvantage that for a determination of the instantaneous position of the scanning unit relative to the scale member, e.g., at the beginning of the measuring work or after an interruption of the operation, it is necessary to move the scanning unit to at least one reference mark and in most cases to at least two reference marks and then to move the scanning unit back to the previously reached position to be detected. That procedure involves more operating and adjusting work and when the measuring systems are used, e.g., in combination with control systems of machine tools and robots, it will be necessary to ensure that any tools provided in the processing machine and engaging the workpiece and/or any robot parts for performing movements which are to be monitored by the measuring system must not damage themselves and/or the workpiece concerned during that adjustment which is required for the determination of the absolute position.